For the majority of last 100 years, energy metering has been almost exclusively performed using electromechanical meters. These meters are easily identified by a large spinning disk in their center rotating at a rate proportional to the rate of energy usage (power). The basic function of these traditional meters is that an electromechanical transducer generates a rotational force in response to the magnitude of voltage and current passing through the sensors. This force then rotates a mechanical counter that is used to store and display the net energy used by the household or business for which the meter is used. Drawbacks to the electromechanical meter include limited accuracy (1%-2%) and limited functionality.
Solid-state energy meters employ integrated circuit (IC) technology in order to accurately measure voltage and current which are then used to determine energy usage. While the solid-state meters have provided higher accuracy than the electromechanical meters since they were first developed, they were not always cost-competitive. However over the last decade, solid-state meters have ramped in volume resulting in a significant reduction in cost. The pricing of a solid-state meter is now the same or less than the electromechanical variants while providing many more features.
Solid-state energy meters have, along with the cost benefits and improved accuracy relative to electromechanical versions, several valuable additional features. Since the data is almost always stored digitally in a solid-state meter, the energy meter's data can be broadcast or accessed remotely with a modem using wireless, power-line carrier, or phone-line communication. This provides a large benefit to utilities both for “reading” meters and for diagnostic purposes. Another feature available with solid-state energy meters is the ability to charge different usage rates based on time of day (multi-tariff). This allows utilities to set energy costs higher during peak demand, thereby encouraging users to conserve energy during these times. This saves money for both the utility and the user.
A common requirement for solid-stage energy meters is that they keep (real) time in order to provide multi-tariff (time-of-day) billing. As a result, when this requirement is in place, the meter must have a means of operating when power is lost from the main supply; thus a battery backup is required. The battery backup is also required if the meter must be read when the (main) power is down, either using an LCD display (common to solid-state meters) or using a modem.
The cost of a battery is related to its energy storage capacity; the larger the energy stored, the higher the cost. To minimize the added cost to the meter, the power used by the meter when running from the battery must be minimized to enable a smaller, less expensive battery to be used.